The field of this invention is the removal of water-soluble impurities from an aqueous solution containing a water-soluble cellulose ether and a water-soluble impurity.
Water-soluble cellulose ethers are used for their many different properties and are employed in a variety of applications such as as thickeners, binders, film formers, water-retention aids, suspension aids, surfactants, lubricants, protective colloids, or emulsifiers, and are used in a variety of end uses such as foods, cosmetics, pharmaceuticals, latex paints, construction products, or ceramics.
A problem encountered by those that produce and consequently those that use the water-soluble cellulose ethers is the presence of impurities. Typically, cellulose ethers are prepared by reacting alkali cellulose with one or more etherifying agents. The etherifying agents are generally alkylhalides, alkylene oxides, halocarboxylic acids or dialkyl sulfates. The etherifying agents are highly reactive and can form impurities upon reacting with each other, water, or the alkali on the cellulose. The impurities formed in these side reactions must be removed before the cellulose ether can be employed in certain end uses, for example, in the pharmaceutical and food fields.
In view of the fact that many water-soluble cellulose ethers are insoluble in hot water, some of the impurities in the final product can be removed by washing the cellulose ether product in hot water at a temperature above the gel point, i.e., the temperature at which the cellulose ether precipitates. Typically, such a procedure is performed by spreading the cellulose ether product on a large filter, and then spraying the product with hot water. However, an amount of the cellulose ether remains soluble in the hot water. Moreover, for those cellulose ethers which have gel points close to the boiling point of water at standard pressure, a larger portion of the cellulose ether remains soluble. The portion of the cellulose ether which remains soluble is washed away in the hot water wash causing product loss. For example, it is typical to lose about 10 to about 30 percent of low viscosity hydroxypropyl methylcellulose and about 2 to about 10 percent of high viscosity hydroxypropyl methylcellulose The low viscosity species typically experience higher losses because, generally, a larger portion of the low viscosity species is soluble in water at any given temperature.
The problem of product loss in the hot water washing process is aggravated by the fact that a large quantity of water is required to remove the desired amount of impurities. Typically, from about 5 to about 20 parts of water are required to remove one part of the impurities. This large amount of water removes a correspondingly large amount of cellulose ether product along with the impurities. Additionally, when this large amount of water containing the impurities and products is discharged, large costs are incurred to decontaminate the water.
U.S. Pat. No. 4,404,370 discloses that cellulose ethers can be purified in a countercurrent process. In such a process, a suspension of cellulose ether is dropped onto a conveyor belt, and washed continuously with a water/alcohol purifying agent. The purifying agent is sprayed onto the cellulose ether from a number of ports, and filtered off. Some of the filtered purifying agent is recycled and used as the purifying agent at a different port. Such a process has the advantage of efficient use of the purifying agent, but does not provide a means for controlling other process parameters such as temperature and pressure, and requires the addition of a purifying agent which is different from the suspension agent.
U.S. Pat. No. 4,581,446 teaches a process of removing solvent-soluble impurities from a cellulose ether composition. This process comprises providing a functionally effective thickness of a cellulose ether composition in a filtration device. The cellulose ether composition is contacted with an extracting solution comprising a solvent for the impurities, in a manner such that the extracting solution can migrate through the cellulose ether composition and through the filtration device. The extracting solution is allowed to migrate through the thickness of the cellulose ether composition such that substantial thickness of the cellulose ether is maintained in the filtration device, and a gradient impurity concentration is effected through the thickness of the cellulose ether composition. This process, however, is expensive and tedious to operate and maintain and does not allow for recovery of the low molecular weight cellulose ether losses as a separate product.
U.S. Pat. No. 4,672,113 teaches a process for the purification treatment of liquid reaction and washing media obtained in the preparation of cellulose ethers. This process includes the ultrafiltration of a residue of a distillation procedure, wherein the residue substantially comprises salt, a cellulose constituent and medium- and higher-molecular weight ethylene glycol derivatives. The permeate which is collected from the ultrafiltration can be fed directly to a waste-water purifying plant. The concentrate which is collected from the ultrafiltration substantially comprises the salt and the cellulose constituents in approximately equal proportions.
In view of the disadvantages of known methods, it is desirable to provide a process for removing water-soluble impurities from an aqueous solution containing a water-soluble cellulose ether and a water-soluble impurity in an efficient, versatile system, in which substantial amounts of the impurities are removed without experiencing substantial cellulose ether product losses.
The present invention solves this problem by providing a process which removes water-soluble impurities from an aqueous cellulose ether solution thereby generating a concentrated, purified aqueous cellulose ether solution and an aqueous impurity solution.